Showing papers on "Phase (matter) published in 2011"

Electrochemical investigation of the P2–NaxCoO2 phase diagram.

Abstract: Sodium layered oxides NaxCoO2 form one of the most fascinating low-dimensional and strongly correlated systems; in particular P2–NaxCoO2 exhibits various single-phase domains with different Na+/vacancy patterns depending on the sodium concentration. Here we used sodium batteries to clearly depict the P2–NaxCoO2 phase diagram for x≥0.50. By coupling the electrochemical process with an in situ X-ray diffraction experiment, we identified the succession of single-phase or two-phase domains appearing on sodium intercalation with a rather good accuracy compared with previous studies. We reported new single-phase domains and we underlined the thermal instability of some ordered phases from an electrochemical study at various temperatures. As each phase is characterized by the position of its Fermi level versus the Na+/Na couple, we showed that the synthesis of each material, even in large amounts, can be carried out electrochemically. The physical properties of the as-prepared Na1/2CoO2 and Na2/3CoO2 ordered phases were characterized and compared. Electrochemical processes are confirmed to be an accurate route to precisely investigate in a continuous way such a complex system and provide a new way to synthesize materials with a very narrow existence range.

Identifying surface structural changes in layered Li-excess nickel manganese oxides in high voltage lithium ion batteries: A joint experimental and theoretical study

Abstract: High voltage cathode materials Li-excess layered oxide compounds Li[NixLi1/3−2x/3Mn2/3−x/3]O2 (0 < x < 1/2) are investigated in a joint study combining both computational and experimental methods. The bulk and surface structures of pristine and cycled samples of Li[Ni1/5Li1/5Mn3/5]O2 are characterized by synchrotron X-Ray diffraction together with aberration corrected Scanning Transmission Electron Microscopy (a-S/TEM). Electron Energy Loss Spectroscopy (EELS) is carried out to investigate the surface changes of the samples before/after electrochemical cycling. Combining first principles computational investigation with our experimental observations, a detailed lithium de-intercalation mechanism is proposed for this family of Li-excess layered oxides. The most striking characteristics in these high voltage high energy density cathode materials are 1) formation of tetrahedral lithium ions at voltage less than 4.45 V and 2) the transition metal (TM) ions migration leading to phase transformation on the surface of the materials. We show clear evidence of a new spinel-like solid phase formed on the surface of the electrode materials after high-voltage cycling. It is proposed that such surface phase transformation is one of the factors contributing to the first cycle irreversible capacity and the main reason for the intrinsic poor rate capability of these materials.

Interfacial phase-change memory

Abstract: Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating1 or some other excitation process2,3,4,5. For example, switching the composite Ge2Sb2Te5 (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude6, and also increases reflectivity across the visible spectrum7,8. Moreover, phase-change memory based on GST is scalable9,10,11, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses1,6. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process12,13. In particular, aligning the c-axis of a hexagonal Sb2Te3 layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb2Te3 interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds. Limiting the movement of Ge atoms to one dimension improves the performance of data-storage devices based on the Ge–Sb–Te material system.

Aqueous Liquid Crystals of Graphene Oxide

Abstract: The formation of liquid crystals (LCs) is the most viable approach to produce macroscopic, periodic self-assembled materials from oriented graphene sheets. Herein, we have discovered that well-soluble and single-layered graphene oxide (GO) sheets can exhibit nematic liquid crystallinity in water and first established their isotropic−nematic solid phase diagram versus mass fraction and salt concentration. The zeta potential of GO dispersion is around −64 mV, and its absolute value decreases with increasing salt concentration, implying that the electrostatic repulsive force between negatively charged GO sheets is the dominant interaction in the system of GOLCs and also explaining the salt-dependent phase behavior. For single-layer GO sheets with average diameter of 2.1 μm and polydispersity index of 83%, the isotropic−nematic phase transition occurs at a mass concentration of ∼0.025%, and a stable nematic phase forms at ∼0.5%. Rheological measurements showed that GO aqueous dispersions performed as typical ...

The spitzer ice legacy: ice evolution from cores to protostars

Abstract: Ices regulate much of the chemistry during star formation and account for up to 80% of the available oxygen and carbon. In this paper, we use the Spitzer c2d Legacy ice survey, complimented with data sets on ices in cloud cores and high-mass protostars, to determine standard ice abundances and to present a coherent picture of the evolution of ices during low- and high-mass star formation. The median ice composition H_(2)O:CO:CO_2:CH_(3)OH:NH_3:CH_4:XCN is 100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low- and high-mass protostars, respectively, and 100:31:38:4:-:-:- in cloud cores. In the low-mass sample, the ice abundances with respect to H_(2)O of CH_4, NH_3, and the component of CO_2 mixed with H_(2)O typically vary by <25%, indicative of co-formation with H_(2)O. In contrast, some CO and CO_2 ice components, XCN, and CH3OH vary by factors 2-10 between the lower and upper quartile. The XCN band correlates with CO, consistent with its OCN– identification. The origin(s) of the different levels of ice abundance variations are constrained by comparing ice inventories toward different types of protostars and background stars, through ice mapping, analysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on the analysis, the first ice formation phase is driven by hydrogenation of atoms, which results in an H_(2)O-dominated ice. At later prestellar times, CO freezes out and variations in CO freezeout levels and the subsequent CO-based chemistry can explain most of the observed ice abundance variations. The last important ice evolution stage is thermal and UV processing around protostars, resulting in CO desorption, ice segregation, and the formation of complex organic molecules. The distribution of cometary ice abundances is consistent with the idea that most cometary ices have a protostellar origin.

Gas uptake and chemical aging of semisolid organic aerosol particles

Abstract: Organic substances can adopt an amorphous solid or semisolid state, influencing the rate of heterogeneous reactions and multiphase processes in atmospheric aerosols. Here we demonstrate how molecular diffusion in the condensed phase affects the gas uptake and chemical transformation of semisolid organic particles. Flow tube experiments show that the ozone uptake and oxidative aging of amorphous protein is kinetically limited by bulk diffusion. The reactive gas uptake exhibits a pronounced increase with relative humidity, which can be explained by a decrease of viscosity and increase of diffusivity due to hygroscopic water uptake transforming the amorphous organic matrix from a glassy to a semisolid state (moisture-induced phase transition). The reaction rate depends on the condensed phase diffusion coefficients of both the oxidant and the organic reactant molecules, which can be described by a kinetic multilayer flux model but not by the traditional resistor model approach of multiphase chemistry. The chemical lifetime of reactive compounds in atmospheric particles can increase from seconds to days as the rate of diffusion in semisolid phases can decrease by multiple orders of magnitude in response to low temperature or low relative humidity. The findings demonstrate that the occurrence and properties of amorphous semisolid phases challenge traditional views and require advanced formalisms for the description of organic particle formation and transformation in atmospheric models of aerosol effects on air quality, public health, and climate.

Phase behavior and properties of the liquid-crystal dimer 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane: A twist-bend nematic liquid crystal

Abstract: The liquid-crystal dimer 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and (2)H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.

Polyelectrolyte complexes: Bulk phases and colloidal systems

Abstract: When aqueous solutions of polycations and polyanions are mixed, polyelectrolyte complexes form. These are usually insoluble in water, so that they separate out as a new concentrated polymer phase, called a complex coacervate. The behavior of these complexes is reviewed, with emphasis on new measurements that shed light on their structural and mechanical properties, such as cohesive energy, interfacial tension, and viscoelasticity. It turns out that stoichiometric complexes can be considered in many respects as pseudo-neutral, weakly hydrophobic polymers, which are insoluble in water, but become progressively more soluble as salt is added. In fact, the solubility-enhancing effect of salt is quite analogous to that of temperature for polymers in apolar solvents. Since two-phase systems can be prepared in colloidal form, we also discuss several kinds of colloids or 'microphases' that can arise due to polyelectrolyte complexation, such as thin films, 'zipper' brushes, micelles, and micellar networks. A characteristic feature of these charge-driven two-phase systems is that two polymeric ingredients are needed, but that some deviation from strict stoichiometry is tolerated. This turns out to nicely explain how and when the layer-by-layer method works, how a 'leverage rule' applies to the density of the 'zipper brush', and why soluble complexes or micelles appear in a certain window of composition. As variations on the theme, we discuss micelles with metal ions in the core, due to incorporation of supramolecular coordination polyelectrolytes, and micellar networks, which form a new kind of physical gels with unusual properties.

Colloids and the Depletion Interaction

Abstract: Preface 1 Introduction 1.1 Colloidal interactions 1.2 The Von Guericke force 1.3 Depletion 1.4 Manifestations of depletion effects in colloid + polymer mixtures 1.5 Historical overview on depletion 1.5.1 Experimental observations before the 1950s 1.5.2 Attractive forces in Nagoya 1.5.3 Systematic studies after AO 1.6 Outline of this book 2 Depletion Interaction 2.1 Depletion interaction due to penetrable spheres 2.1.1 Depletion interaction between two flat plates 2.1.2 Depletion interaction between two spheres 2.2 Depletion interaction due to ideal polymers 2.2.1 Depletion interaction between two flat plates 2.2.2 Interaction between two spheres 2.3 Depletion interaction due to excluded volume polymers 2.3.1 Characteristic length scales in polymer solutions 2.3.2 Osmotic pressure of polymer solutions 2.3.3 Depletion thickness due to excluded volume polymers 2.3.4 Evaluation of the depletion interaction due to excluded volume polymers 2.4 Depletion interaction due to spheres 2.4.1 Concentration profiles near a hard wall and between two hard walls 2.4.2 Depletion interaction between two flat plates 2.4.3 Depletion interaction between two (big) spheres 2.5 Depletion interaction due to rods 2.5.1 Depletion interaction between two flat plates 2.5.2 Interaction between two (big) colloidal spheres using the Derjaguin approximation 2.6 Depletion interaction due to disks 2.6.1 Depletion interaction between two flat plates 2 .6.2 Interaction bet ween two (big) colloidal spheres using the Derjaguin approximation 2.7 Measurements of depletion interactions 2.7.1 Atomic force microscope 2.7.2 Total internal eflection microscopy 2.7.3 Optical tweezers 3 Phase transitions of hard spheres plus depletants basics 3.1 Introduction -colloid/atom analogy 3.2 The hard sphere fluid-crystal transition 3.3 Free volume theory Appendix 3.1. Statistical Mechanical derivation of the Free Volume Theory 4 Stability of colloid-polymer mixtures 4.1 Experimental state diagrams of colloid-polymer mixtures 4.2 Phase behaviour of colloid + ideal polymer mixtures 4.3 Phase behaviour of sphere plus interacting polymer mixtures GFVT 4.3.1 Depletion thickness and osmotic pressure 4.3.2 Protein Limit 4.4 Non-equilibrium phenomena 4.4.1 Unmixing kinetics 4.4.2 Aggregation and gelation 4.4.3 Depletion effects on colloidal glasses 5 Phase transitions of hard spheres plus colloids 5.1 Free volume theory for big plus small hard spheres 5.2 Phase behavior of mixed spheres 5.2.1 Phase separation in binary mixtures differing only in diameter 5.2.2 Mixtures of latex particles and micelles 5.2.3 Oil-in-water emulsion particles and micelles of the stabilizing surfactant 5.3 free volume theory for sphere-rod mixtures 5.4 Phase behaviour of sphere-rod mixtures 6 Suspensions of rod-like colloids plus polymers 6.1 Onsager theory of the isotropic-nematic transition 6.2 Scaled particle theory of the isotropic-nematic transition 6.3 Isotropic-nematic phase behaviour of rods plus phs 6.4 I-N phase behaviour of rods plus polymers 6.4.1 Rod-like colloids plus ideal polymers 6.4.2 Rod-like colloids plus interacting polymers 6.5 Experiments on rod/polymer mixtures 6.5.1 Stiff virusparticles + polymer 6.5.2 Cellulose nanocrystals + polymer 6.5.3 Sterically stabilized colloidal boehmite rods + polymer 6.6 Rod/polymer mixtures: full phase diagrams. 6.7 Concluding remarks. List of symbols Index

Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge

Abstract: Using a novel electrochemical phase-field model, we question the common belief that LiXFePO4 nanoparticles always separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO4 cathodes.

Evolving morphotropic phase boundary in lead-free (Bi1/2Na1/2)TiO3–BaTiO3 piezoceramics

Abstract: The correlation between structure and electrical properties of lead-free (1−x)(Bi1/2Na1/2)TiO3–xBaTiO3 (BNT-100xBT) polycrystalline piezoceramics was investigated systematically by in situ synchrotron diffraction technique, combined with electrical property characterization It was found that the morphotropic phase boundary (MPB) between a rhombohedral and a tetragonal phase evolved into a morphotropic phase region with electric field In the unpoled material, the MPB was positioned at the transition from space group R3m to P4mm (BNT-11BT) with optimized permittivity throughout a broad single-phase R3m composition regime Upon poling, a range of compositions from BNT-6BT to BNT-11BT became two-phase mixture, and maximum piezoelectric coefficient was observed in BNT-7BT It was shown that optimized electrical properties are related primarily to the capacity for domain texturing and not to phase coexistence

Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy

Abstract: Surface phenomena at the air–water interface are of vital importance in many situations, from oceanography to atmospheric and environmental chemistry. An unanswered question in the field is how thin is the interfacial region — or how soon do the properties of bulk liquid water reappear as the interface is crossed. Using spectroscopy to probe the 'free OD' vibrational mode of water molecules with an oxygen–deuterium bond protruding from the surface and theoretical modelling to interpret the results, Stiopkin et al. find that water molecules straddling the interface form hydrogen bonds that are only slightly weaker than those in bulk water. This suggests a remarkably rapid onset of bulk-phase behaviour, and an extremely short 'healing length' for the interface on crossing from the air into the water phase. The air–water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth’s surface and strongly affects atmospheric, aerosol and environmental chemistry. The air–water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short ‘healing length’ of about 3 angstroms (1 A = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers1,2,3. However, direct experimental evidence has been elusive owing to the difficulty of depth-profiling the liquid surface on the angstrom scale. Most physical, chemical and biological properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are determined by its hydrogen-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the hydrogen-bond strength4,5,6. Here we report a combined experimental and theoretical study of the air–water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the ‘free OD’ transition found only in the topmost water layer. By using deuterated water and isotopic dilution to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramolecular coupling to the stretching of the other OD group on the same molecule. The other OD stretch frequency indicates the strength of one of the first hydrogen bonds encountered at the surface; this is the donor hydrogen bond of the water molecule straddling the interface, which we find to be only slightly weaker than bulk-phase water hydrogen bonds. We infer from this observation a remarkably fast onset of bulk-phase behaviour on crossing from the air into the water phase.

Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge

Abstract: Using a novel electrochemical phase-field model, we question the common belief that LixFePO4 nanoparticles separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO4 cathodes.

Active jamming: Self-propelled soft particles at high density

Abstract: We study numerically the phases and dynamics of a dense collection of self-propelled particles with soft repulsive interactions in two dimensions. The model is motivated by recent in vitro experiments on confluent monolayers of migratory epithelial and endothelial cells. The phase diagram exhibits a liquid phase with giant number fluctuations at low packing fraction φ and high self-propulsion speed v(0) and a jammed phase at high φ and low v(0). The dynamics of the jammed phase is controlled by the low-frequency modes of the jammed packing.

Coherency strain and the kinetics of phase separation in LiFePO4

Abstract: A theoretical investigation of the effects of elastic coherency on the thermodynamics, kinetics, and morphology of intercalation in single LiFePO4 nanoparticles yields new insights into this important battery material Anisotropic elastic stiffness and misfit strains lead to the unexpected prediction that low-energy phase boundaries occur along {101} planes, while conflicting reports of phase boundary orientations are resolved by a partial loss of coherency in the {100} direction Elastic relaxation near surfaces leads to the formation of a striped morphology, whose characteristic length scale is predicted by the model and yields an estimate of the interfacial energy The effects of coherency strain on solubility and galvanostatic discharge are studied with a reaction-limited phase-field model, which quantitatively captures the influence of misfit strain, particle size, and temperature on solubility seen in experiments Coherency strain strongly suppresses phase separation during discharge, which enhances rate capability and extends cycle life The effects of elevated temperature and the feasibility of nucleation are considered in the context of multi-particle cathodes

Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions

Abstract: The dispersion microstructure and rheological properties of aqueous sulfonated cellulose nanocrystal (CNC) suspensions have been investigated. Between 3.07 and 10.4 vol % the suspensions phase separated into liquid crystalline and isotropic domains. At 12.1 vol %, no isotropic phase was visible, and the samples had the fingerprint texture characteristic of a cholesteric liquid crystal. Below 35 °C, temperature had little influence on rheology and phase behavior. However, between 35 and 40 °C there was a significant change in both the fraction of isotropic phase and the rheological properties. In contrast to many lyotropic suspensions, the steady shear viscosity did not go through a maximum with increasing concentration. Maxima were observed for complex viscosity, storage modulus, and loss modulus at concentrations that appeared fully liquid crystalline. Time–concentration superposition was successful for the loss modulus but not the storage modulus. This suggests that the interface in biphasic samples aff...

Bulk heterojunction photovoltaic active layers via bilayer interdiffusion.

Abstract: To better understand the physics of the photoactive layer in the organic photovoltaic devices, it is necessary to gain a quantitative understanding of the morphology and the manner in which it develops. A key element in the kinetics associated with the structure development is the interdiffusion of the components. To that end we used P3HT/PCBM bilayers as a model to investigate the interdiffusion of the components and its role in the development of the morphology. A detailed description of the diffusion behavior and the morphology developed from a layer of P3HT in contact with a layer of PCBM during thermal annealing is given. Amorphous P3HT and PCBM are shown to be highly miscible and PCBM can penetrate into the P3HT layer through the P3HT amorphous region and form the bulk heterojunction structure within a few seconds of annealing at 150 °C. The results indicated that one phase is an ordered P3HT domain and the other phase is the mixture of amorphous P3HT and PCBM which is not consistent with a phase separation of the components by a spinodal decomposition mechanism.

Sequential Cation Exchange in Nanocrystals: Preservation of Crystal Phase and Formation of Metastable Phases

Abstract: We demonstrate that it is possible to convert CdSe nanocrystals of a given size, shape (either spherical or rod shaped), and crystal structure (either hexagonal wurtzite, i.e., hexagonal close packed (hcp), or cubic sphalerite, i.e., face-centered cubic (fcc)), into ZnSe nanocrystals that preserve all these characteristics of the starting particles (i.e., size, shape, and crystal structure), via a sequence of two cation exchange reactions, namely, Cd2+ ⇒Cu+ ⇒Zn2+. When starting from hexagonal wurtzite CdSe nanocrystals, the exchange of Cd2+ with Cu+ yields Cu2Se nanocrystals in a metastable hexagonal phase, of which we could follow the transformation to the more stable fcc phase for a single nanorod, under the electron microscope. Remarkably, these metastable hcp Cu2Se nanocrystals can be converted in solution into ZnSe nanocrystals, which yields ZnSe nanocrystals in a pure hcp phase.

Phase transitions in ferroelectric silicon doped hafnium oxide

Abstract: We investigated phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) by temperature dependent polarization and x-ray diffraction measurements. If heated under mechanical confinement, the orthorhombic ferroelectric phase reversibly transforms into a phase with antiferroelectric behavior. Without confinement, a transformation into a monoclinic/tetragonal phase mixture is observed during cooling. These results suggest the existence of a common higher symmetry parent phase to the orthorhombic and monoclinic phases, while transformation between these phases appears to be inhibited by an energy barrier.

Interaction of Microstructure and Interfacial Adhesion on Impact Performance of Polylactide (PLA) Ternary Blends

Abstract: Polyactide (PLA) was blended with an ethylene/n-butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer and a zinc ionomer of ethylene/methacrylic acid (EMAA-Zn) copolymer. The phase morphology of the resulting ternary blends and its relationship with impact behaviors were studied systematically. Dynamic vulcanization of EBA-GMA in the presence of EMAA-Zn was investigated by torque rheology, and its cross-link level was evaluated by dynamic mechanical analysis. Reactive compatibilization between PLA and EBA-GMA was studied using Fourier transform infrared spectroscopy. The dispersed domains in the ternary blends displayed a “salami”-like phase structure, in which the EMAA-Zn phase evolved from occluded subinclusions into continuous phase with decrease in the EBA-GMA/EMAA-Zn ratio. An optimum range of particle sizes of the dispersed domains for high impact toughness was identified. Also, the micromechanical deformation process of these ternary blends was also investigated by observation of the impact-fra...

Amorphous Carbon Coated High Grain Boundary Density Dual Phase Li4Ti5O12‐TiO2: A Nanocomposite Anode Material for Li‐Ion Batteries

Abstract: This work introduces an effective, inexpensive, and large-scale production approach to the synthesis of a carbon coated, high grain boundary density, dual phase Li4Ti5O12-TiO2 nanocomposite anode material for use in rechargeable lithium-ion batteries. The microstructure and morphology of the Li4Ti5O12-TiO2-C product were characterized systematically. The Li4Ti5O12-TiO2-C nanocomposite electrode yielded good electrochemical performance in terms of high capacity (166 mAh g−1 at a current density of 0.5 C), good cycling stability, and excellent rate capability (110 mAh g−1 at a current density of 10 C up to 100 cycles). The likely contributing factors to the excellent electrochemical performance of the Li4Ti5O12-TiO2-C nanocomposite could be related to the improved morphology, including the presence of high grain boundary density among the nanoparticles, carbon layering on each nanocrystal, and grain boundary interface areas embedded in a carbon matrix, where electronic transport properties were tuned by interfacial design and by varying the spacing of interfaces down to the nanoscale regime, in which the grain boundary interface embedded carbon matrix can store electrolyte and allows more channels for the Li+ ion insertion/extraction reaction. This research suggests that carbon-coated dual phase Li4Ti5O12-TiO2 nanocomposites could be suitable for use as a high rate performance anode material for lithium-ion batteries.

Superionic state in double-layer capacitors with nanoporous electrodes

Abstract: In recent experiments (Chmiola et al 2006 Science 313 1760; Largeot et al 2008 J. Am. Chem. Soc. 130 2730) an anomalous increase of the capacitance with a decrease of the pore size of a carbon-based porous electric double-layer capacitor has been observed. We explain this effect by image forces which exponentially screen out the electrostatic interactions of ions in the interior of a pore. Packing of ions of the same sign becomes easier and is mainly limited by steric interactions. We call this state 'superionic' and suggest a simple model to describe it. The model reveals the possibility of a voltage-induced first order transition between a cation(anion)-deficient phase and a cation(anion)-rich phase which manifests itself in a jump of capacitance as a function of voltage.

The Role of a Bilayer Interfacial Phase on Liquid Metal Embrittlement

Abstract: Intrinsically ductile metals are prone to catastrophic failure when exposed to certain liquid metals, but the atomic-level mechanism for this effect is not fully understood. We characterized a model system, a nickel sample infused with bismuth atoms, by using aberration-corrected scanning transmission electron microscopy and observed a bilayer interfacial phase that is the underlying cause of embrittlement. This finding provides a new perspective for understanding the atomic-scale embrittlement mechanism and for developing strategies to control the practically important liquid metal embrittlement and the more general grain boundary embrittlement phenomena in alloys. This study further demonstrates that adsorption can induce a coupled grain boundary structural and chemical phase transition that causes drastic changes in properties.

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

Abstract: We designed a Pb-free pseudo-binary system, Ba(Sn012Ti088)O3-x(Ba07Ca03)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 � 530 pC/N at room temperature In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys Rev Lett 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics V C 2011 American Institute of Physics

On the superior hot hardness and softening resistance of AlCoCrxFeMo0.5Ni high-entropy alloys

Abstract: Effects of Cr content on microstructures and hot hardness of AlCoCrxFeMo0.5Ni high-entropy alloys (x = 0–2.0) were investigated. The cast microstructure of AlCoCrxFeMo0.5Ni consists of B2 and σ phases, both being multi-element solid solutions. Increasing Cr content increases the volume fraction of σ phase and causes the matrix phase of the dendrite to change from B2 phase to σ phase. The alloy hardness increases from Hv 601 at x = 0 to Hv 867 at x = 2.0 as a result of increasing amount of hard σ phase. A phase diagram for the AlCoCrxFeMo0.5Ni alloy system is constructed based on SEM, HTXRD and DTA analyses, providing useful information for understanding and designing high-entropy alloys. All the AlCoCrxFeMo0.5Ni alloys possess higher hot-hardness level than that of Ni-based superalloys, In 718 and In 718 H, from room temperature to 1273 K. Cr-1.5 and Cr-2.0 alloys exhibit a transition temperature higher than that of Co-based alloy T-800 by about 200 K and have respective hardness values, Hv 374 and Hv 450 at 1273 K, being much higher than those, around Hv 127, of In 718 and In 718 H. The mechanism of larger strengthening and softening resistance is related with B2 and σ phases, both having multi-principal-element effect. The AlCoCrxFeMo0.5Ni alloy system has a potential in high-temperature applications.

Methylene-linked liquid crystal dimers and the twist-bend nematic phase

Abstract: The transitional properties of three methylene-linked liquid crystal dimers are reported, namely, 1,5-bis(4-cyanoanilinebenzylidene-4′-yl)pentane (CN-5-CN), 1,5-bis(4-methoxyanilinebenzylidene-4′-yl)pentane (1O-5-O1), and 1,5-bis(4-ethoxyanilinebenzylidene-4′-yl)pentane (2O-5-O2). Each dimer exhibits two monotropic mesophases. The higher temperature mesophase is a normal nematic phase while the lower temperature phase is assigned as a twist-bend nematic phase. The assignment of the twist-bend nematic phase was based on the strong similarities in the optical textures observed to those reported recently for a structurally similar dimer. The complete miscibility of the mesophases exhibited by CN-5-CN and 1O-5-O1 has been established. The analogous hexamethylene-linked dimers exhibit only the normal nematic phase as do the corresponding ether-linked dimers. A review of the literature reveals another five methylene-linked odd-membered dimers that exhibit a nematic–nematic transition and, in each, the lower tem...